That dielectric heating of musculoskeletal tissue is more efficacious and more efficiently accomplished by contact applicators was established by Kantor, U.S. Pat. No. 4,108,147.
Subsequent improvements deal with broad band tuning to accomplish efficient transfer of microwave energy from applicator into tissue over a wide band of frequencies, and cooling using air as well as water, or convective as well as conductive.
A slotted, metallic cover over the radiating aperature of a waveguide applicator was the subject of the Potzl patent, U.S. Pat. No. 3,065,752.
Vaguine in U.S. Pat. No. 4,446,874 made claims concerning coupling and tuning involving discoupling input coupling of the magnetic loop whereby the frequency of operation and input match are adjusted.
The design procedures cited in the referenced patents were not viable because these procedures did not provide for compensation and control of evanescent modes in the waveguide applicator. These modes exist principally in the area of the feed that couples microwave energy from the generator via a coaxial cable into the applicator, and on into the patient.
As a result of these modes, the guide wavelength is not equal to the free space wavelength at the frequency of operation, i.e., the TEM mode referred to by Kantor in U.S. Pat. No. 4,108,147 is not necessarily established. The TEM mode cannot exist in a hollow tube waveguide, nevertheless, uniformity of the electric field across the aperture can be improved when the guide wave length is shortened to that of the free space by the use of partial filling with dielectric material parallel to the narrow wall of the guide.
The reduction of guide wavelength to the free space value is, therefore, a necessary, but not a sufficient condition to accomplish more nearly uniform electric fields across the aperture. Although the guide wavelength may also be shortened by partial filling by dielectric parallel to the broad wall, this does not yield the uniform electric field distribution across the aperture.
The instant invention shows that the length of the dielectric material in the waveguide applicator must also be an appreciable fraction of a wavelength in order to establish the desired guide wavelength and provides for confirmation of the guide wavelength and methods of quality assurance; no methods for confirmation of the guide wavelength or for quality assurance exist in the prior art.
Additionally, this invention establishes the relationship between aperture electric field distribution, guide wavelength, and specific absorption rates at depth sites, none of which is provided for by the prior art.
The instant invention also improves air cooling methods of prior designs which were non-contacting in order to allow egress of air over the patient's skin. The necessary spacing promoted RF leakage as well as tuning variation as the air gap varied with breathing or other motions. RF leakage was not controlled at the point of air ingress and the simple propeller fan mounted within the applicator of the prior design introduces undesirable vibration that modulates the match by alteration of the air gap.
The invention provides for detection of therapeutic response to control dielectric heating during the treatment. Until this invention, no specific individual treatment response has been possible. Prior art was subjective, and at best, the manufacturer provided tabled standards of power and duration for the general population which failed to accomodate for the variations between individuals and were no more than general recommendations. The instant invention makes use of the increase in local blood flow in muscle consequent to local temperature elevation and wave impedance change due to the change in tissue electrical properties, i.e., the instant invention both provokes and detects reactive hyperemia which is the therapeutic response.
The technology of combined microwave heating with sensing has been recognized in other areas. The Furihata patent, U.S. Pat. No. 4,409,993 addressed the need to control dose in an endoscopic device that uses microwave power to heat cancerous tissue to the point of eschar as verified by optical visualization of necrosis. The Converse patent, U.S. Pat. No. 4,312,364, and its progeny, the Carr patents, U.S. Pat. Nos. 4,346,716 and 4,557,272, use microwave radiometry to sense heating from an exogenous microwave source.
The instant invention uniquely provides for both heating and sensing of the therapeutic response through the dual use of an antenna. No prior art utilizes the combined effector/sensor action of the instant invention, using the change in complex permittivity of the target tissue due to local vasodilation, nor were the prior methods based on a closely coupled antenna whereby antenna impedance alterations are used to infer changes in the wave impedance of the tissue secondary to the desired reactive hyperemia. Prior technology did not use the critical guide wavelength, design methods, a radome, or surface cooling incorporated as with the instant invention.